Until 1821, the leader of the United States Army was simply known as the Senior Officer. In 1821 the position became Commanding General. When Winfield Scott assumed command in 1841, the name was changed to General-in-Chief. That was the name used to designate the commander of all armies during the Civil War.

The position was responsible for overseeing the strategic deployment of troops as well as communicating with non-military personnel and the Commander-in-Chief (President of the United States Abraham Lincoln. Winfield Scott served General-in-Chief at the start of the war. Limited physically by 320 lbs. of weight, Scott was also viewed as "living in the past" and feeble-minded. Neither was true. Scott was a sharp, astute military mind until his death after the Civil War. His plan for winning the war, dubbed "Anaconda" by reporters, was the blueprint finally used by the Union to defeat the Confederates.

Scott resigned under pressure following the defeat at Ball's Bluff. Lincoln replaced him with George B. McClellan. Over the next year McClellan proved to be an inadequate communicator for the position and Lincoln removed him but let him continue as the commander of the Army of the Potomac. Following McClellan's removal, Lincoln and Secretary of War Edwin Stanton tried to assume the responsibilities of the General-in-Chief position.

When Stanton and Lincoln realized the position required a military man who was an experienced administrator, they promoted Henry Halleck, who earned a name for himself being in charge of the Western Theater. Lincoln soon grasped the fact that it was not Halleck but Ulysses S. Grant who was responsible for the successes of the Western Theater, but with the Union Army extended along the Mississippi River, he felt it would be best to leave Halleck in the position for the time being.

With Vicksburg safely in Union hands, Lincoln promoted Grant and gave him the General-in-Chief position. Grant chose to stay with the Army of the Potomac and make strategic decisions while Halleck handled the administrative duties of the job. Grant proved to be the General-in-Chief Lincoln had spent the war searching for. With more men and material than the South, Grant was willing to wear the Army of Northern Virginia down man for man. Using all of General Scott's original plan for winning the war, Grant defeated the Confederate Armies of both the Eastern and Western Theaters in just over a year.

In 1903 the title of General-in-Chief was renamed to Army Chief of Staff.